Setting up Webpack manually is a pain, but honestly if one uses create-react-app, everything is done for you and you just start coding your app: you have transpiling of ES6 (at least, I don't know about ES7), you can install and include other packages, etc. etc.

That link preloads mithril in a web playground and you're ready to go in a couple of seconds.
You can even download and continue development locally should the need arise - without the gorilla and the jungle hiding behind it.